Innate airway responses shape permissiveness to human respiratory syncytial virus.

The interaction between human respiratory syncytial virus (HRSV) and the innate immune system has been demonstrated both in vitro and in vivo. Disruption of interferon (IFN) signaling pathways increases susceptibility and permissiveness to HRSV infection, whereas pretreatment of cells with IFN confers (partial) resistance. This suggests that HRSV disease severity is likely influenced by a pre-existing antiviral state of the respiratory epithelium, driven by baseline or primed expression of type I and type III IFNs. Here, we investigated whether prior exposure to respiratory bacteria or viruses alters in vitro susceptibility and permissiveness to HRSV infection by shaping an antiviral state using both immortalized cell lines and airway organoid models. In A549 cells, pre-exposure to S. aureus had the most significant impact by reducing HRSV infection and inducing robust interferon responses. However, this effect was not reproduced in airway organoids. Conversely, sequential virus infection experiments in airway organoids revealed that prior infection with human parainfluenza virus type 3 (HPIV‑3) reduced the spread of subsequent HRSV infection. In addition to interferon signaling this proved to be associated with epithelial damage mediated by HPIV-3 infection. Collectively, these findings show that HRSV susceptibility and permissiveness are influenced by the up- or downregulation of specific anti- and pro-viral factors induced by prior bacterial or viral exposure, together with the maintenance or disruption of epithelial integrity. Understanding these interactions could be crucial when identifying specific risk groups for severe HRSV-associated disease and the development of targeted HRSV interventions.